Phylum Mollusca

Platyhelminthes Nematoda protostomes Rotifera acoelomate Annelida Mollusca pseudocoelomates Arthropoda a eucoelomates Echinodermat Chordata Phylum Mollusca “soft bodied” animals Mollusca Characteristics Body Plan • head-foot: contains sensory organs and muscles • visceral mass: contains digestive, reproductive, circulatory organs • mantle: skin of the dorsal body wall secretes the shell (if there is one), cavity contains gills or lungs shell mantle Visceral mass mantle cavity foot periostracum: conchiolin prismatic layer: calcium carbonate nacreous layer: calcium carbonate and proteins mantle HAM (Hypothetical Ancestral Mollusc) Mantle gonads coelom Shell Nephridium h eart Anus Digestive gland head Gill Mouth foot Nerve cords Radula Nerve collar Intestine Mollusca Characteristics Feeding and Digestion: There are both free living and parasitic forms Most species use a tongue like organ called a radula when feeding radula retractor radula esophagus odontophore retractor radula protractor odontophore odontophore protractor Mollusca Characteristics Feeding and Digestion: Mollusca Characteristics Digestive System • complete with regional specialization digestive gland anus stomach intestine Mollusca Characteristics Gas Exchange • mainly gills, however terrestrial species have evolved lungs axis afferent blood vessel: into gill water efferent blood vessel: out of gill Mollusca Characteristics Gas Exchange gill gill gill foot foot Mollusca Characteristics Circulatory System • open circulatory system (in most classes) • heart and blood sinuses heart coelom Mollusca Characteristics Excretion • most have kidneys (metanephridia) •tubules connecting pericardial cavity (coelom) and nephridiopore nephridiopore Mollusca Characteristics Nervous system nerve ring pedal nerve cord visceral nerve cord Mollusca Characteristics Reproduction • monoecious and dioecious species exist • usually internal fertilization • indirect development with the presence of a trochophore larva (link to annelids), and in most cases also a veliger larva trochophore veliger ovisacs Lampsilis ovata The juvenile stages of some bivalves are parasitic Glochidia: larval bivalves that are parasitic on fish gills ovisacs Lampsilis ovata Some freshwater bivalves have evolved ways of attracting hosts for their larvae. Some species place their larva in a lure called a superconglutinate http://courses.smsu.edu/mcb095f/gallery/L_perovalis/lampsilis_perovalis.htm Molluscan Radiation • There are approximately 128,000 living species in phylum Mollusca (35,000 are extinct) • The great morphological diversity is the result of elaboration on the basic body plan (HAM) HAM (Hypothetical Ancestral Mollusc) Mantle gonads coelom Shell Nephridium h eart Anus Digestive gland head Gill Mouth foot Nerve cords Radula Nerve collar Intestine Molluscan Radiation Gastropoda • torsion Scaphopoda • coiling of the shell • ventral shell fusion Polyplacophora • shell consists of 8 plates HAM Cephalopoda • lobed foot Bivalvia • highly developed head • bivalved shell • shell reduced or lost • dorsal hinge Phylum Mollusca Class Gastropoda Class Bivalvia Class Cephalopoda Class Polyplacophora Class Scaphopoda Class Gastropoda snails and slugs Torsion • twisting of the visceral mass through a 180° rotation •1st 90° rotation usually occurs at the veliger stage •the 2nd 90° rotation usually takes longer and occurs later • after torsion, the anus and mantle cavity end up over the head • this poses a serious fouling problem and many gastropods have lost their right gills, kidneys and heart auricles mouth mouth anus gills gills anus anus gills Larval torsion anus mouth Coiling Apex of shell extends out making the Planospiral shell shell more compact Conispiral shell Shell shifts over body for better weight distribution Class Gastropoda Subclass Prosobranchia Subclass Subclass Pulmonata Opisthobranchia Class Gastropoda Subclass Prosobranchia • aquatic snails (marine and freshwater) • have undergone torsion and most have undergone shell coiling • have gills • all have shells • use radula for feeding; can be herbivores, carnivores, or detritivores In some, mantle extends over shell Class Gastropoda Subclass Prosobranchia • snails often lay eggs in protective capsules Class Gastropoda Subclass Opisthobranchia • sea hares and sea slugs (nudibranchs) • have undergone detorsion • have gills • sea hares have a reduced shell • sea slugs have no shell Detorsion mouth anus gills They are missing the right gill, kidney and heart auricle Class Gastropoda Subclass Opisthobranchia • sea hares and sea slugs (nudibranchs) Class Gastropoda Subclass Opisthobranchia • they usually crawl along the bottom but many can also swim by using their modified mantle as “wings” Class Gastropoda Subclass Opisthobranchia • all are monoecious • sea hares often form “mating chains” acts as female acts as act as both males only male only and females simultaneously Class Gastropoda Subclass Opisthobranchia • sea slugs often lay “ribbons” of eggs which stick to the substrate Class Gastropoda Subclass Pulmonata • land snails and land slugs • have lungs • land slugs have undergone detorsion and have lost their shell Class Bivalvia (class Pelecypoda) the “bivalves” Class Bivalvia • clams, mussels, oysters, scallops • use gills for respiration and filter feeding • shell is modified into a bivalved shell connected by muscles and ligaments • no head (reduced sensory organs), no radula • foot can be modified for digging dorsal umbo -the oldest part of the shell anterior posterior ridges show where mantle has laid down shell ventral siphons foot labial palps gills excurrent siphon Incurrent siphon mouth foot anus intestine Zebra Mussels: Dreissena polymorpha Native to Asia, introduced to the North America in ballast water Consequences of zebra mussel invasion: 1. Decrease in phytopalnkton (increase in water clarity) 2. Change lake food webs 3. Out compete native mussels 4. Cause physical damage to water intake pipes Richard E. Young, Michael Vecchione and Katharina M. Mangold Class Cephalopoda the squids, octopus, nautilus, and cuttlefish Class Cephalopoda • shell is present, reduced, or lost • all are predacious with beak-like jaws • highly developed head and sensory organs (very intelligent) • closed circulatory system • swim via jet propulsion • foot is lobed and forms tentacles • direct development (no larvae) Class Cephalopoda Nautilus: • have a chambered shell that aids in maintaining buoyancy • chambers are filled with gas chambers siphuncle (cord of tissue connected to visceral mass) Class Cephalopoda Squids: • have a reduced, internal shell shell called the pen Cuttlefish: • have a reduced, internal shell called a cuttlefish bone Class Cephalopoda Octopus: • have lost the shell completely • most intelligent invertebrate Class Cephalopoda Reproduction • male transfers a spermatophore to female using a special pair of tentacles • female seals herself up in a den to lay eggs which she attaches to the top of the den • she cares for these eggs (in some species up to 6.5months) • after the eggs hatch she usually dies Class Cephalopoda • the largest giant squid found to date have been 18 meters in length • they live in the deep sea • sperm whales (~20m in length) are their major predators Class Polyplacophora the “chitons” Class Polyplacophora • have 8 rows of articulating plates • use radula to graze algae on substrate • mantle forms a girdle around plate edge • no veliger larvae girdle plates Class Scaphopoda the “tusk shells” Class Scaphopoda • shell is modified into hollow tube that is open at both ends • the mantle wraps completely around the visceral mass • sessile and feed in sediment located above sand located below sand feeding tentacles.

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